1. Field of the Invention
The present invention generally relates to CMOS structures and, more particularly, to a method of forming source/drain electrodes on thin SOI by employing an in situ doped amorphous silicon epitaxy process.
2. Background Description
Complementary Metal Oxide Semiconductors (CMOS) are electronic components used for Random Access Memory (RAM)and fast data switching. CMOS semiconductors are made of two complementary metal-oxide field effect transistors for high speed and low power use. While many designs have been developed, Silicon On Insulator has recently been of interest due to the high capacity of silicon for high-performance structure fabrication having the channel controllable with low gate voltages.
As silicon film thickness of SOI is reduced, for high-performance CMOS fabrication, it becomes necessary to increase the thickness of source/drain regions above the SOI thickness. This principally results from the fact that, as the SOI film becomes thinner, there exists a reduced amount of Si material from which to form silicide for source/drain contacts. Additionally, thinner source/drain regions can degrade on-current due to increased series resistance.
In particular, when the SOI wafer is 500 xc3x85 and below, it is necessary to build raised source/drain (RSD) structures for silicided contact formation. Source/Drain series resistance is affected by the RSD layer geometry and the doping level in the layer. This is particularly difficult when dealing with PFET and NFET devices because the difference in conductivity between the PFET extension and the NFET halo creates makes it difficult to create a shallow junction and the RSD in sequential order. For sub 0.1 mm CMOS technology on thin SOI, it is necessary to build the RSD layer and shallow junction at the same time. However, the technique of forming the halo and the extension using the dopant diffusion from a solid source interface should be compatible with the requirements for RSD layer formation.
When building an advanced CMOS structure several additional problems are generally encountered. In particular, CMOS processes involving the integration of both P-type and N-type Field Effect Transistors (FETs) cannot employ the same solid source diffusion techniques used to achieve the shallow junction. Attempts have been made to use amorphous silicon as a material for the RSD layer and performing Chemical Mechanical Polishing (CMP) on either the NFET or the PFET. However, for CMOS integration, it is necessary to overcome the problem of forming the interface for solid source diffusion for the P-type and N-type FETs individually without mixing the dopants of the PFET with those of the NFET. There is an existing need for a method of controlled recess formation for NFET and PFET simultaneously.
In one aspect of the present invention, a method of forming source/drain electrodes on thin SOI is provided, which comprises an in situ doped amorphous silicon epitaxy process.
A principal objective in accordance with a preferred embodiment of the present invention, is to contemporaneously achieve a shallow junction for PFET boron extension doping and confine the distribution of NFET boron halo with steep lateral concentration profiles, by using in situ doped amorphous silicon selective epitaxy and appropriately placed removable spacers.
A further objective in accordance with a preferred embodiment of the present invention, is to build the RSD layers contemporaneously with shallow junction formation for the extension region and sharper halo profiles. In the furtherance of this and other objectives, the RSD layers are formed in conjunction with doped amorphous silicon deposition with the opposite types of doping, so that high-performance CMOS devices can be formed on very thin SOI films.
Still another objective in accordance with a preferred embodiment of the present invention is to provide improved controllability of the lateral etch encroachment of silicon under the spacer. In the furtherance of this and other objectives, the method comprises the step of implanting neutral ions such as Ge or Ar into the source/drain regions. The implantation creates an amorphous silicon surface, and leaves a laterally extended amorphous layer under the spacer and a well defined amorphous/crystalline interface. The etch of silicon then extends laterally underneath the spacer, due to the higher etch rate of amorphous silicon and abrupt interface between amorphous and crystalline silicon.
Further objects, features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.